In a typical fiber optic transmission system or network, multiple digital information signals, or channels, are transported among optical terminals by optical fibers. In wavelength division multiplexed (WDM) optical fiber networks, channels each occupying a distinct wavelength of light share the same fiber, thereby effectively increasing the capacity of the fiber. The number of channels per fiber may exceed 32 in the case of dense WDM (DWDM) networks.
The distance between optical terminals of the optical fiber transmission system is limited by transmitter power, receiver sensitivity, and loss and dispersion considerations. Where the distance between end points of an optical fiber transmission system exceeds the maximum distance between optical terminals, amplifiers (also known as repeaters) are provided. As the data rates of optical fiber transmission systems enter the 10 Gigabits per second (Gbps) to 40 Gbps range per channel, it would be prohibitively expensive to convert a multi-wavelength optical input signal into multiple electronic signals prior to amplification and reconverting the amplified electronic signals to a WDM output signal. Therefore, it preferable to use purely optical amplifiers to perform amplification of channels, thereby circumventing the huge potential cost of electronic-to-optical and optical-to-electronic conversion.
At various points along the fiber, there will also be located optical add/drop multiplexer/demultiplexers (ADMs) for introducing (adding) supplementary optical channels coming from another part of the network to the stream of traffic between two optical terminals. The optical ADM also serves to divert (drop) optical channels from the main signal path on the fiber to another part of the network. A selectable but predetermined routing pattern is executed between pairs of input and output channels of the component, i.e., the ADM acts as a switch.
Current optical ADMs are essentially passive components and lack the capability to make decisions related to switching and power control. Consequently, there are specific problems and disadvantages associated with current WDM systems using optical ADMs, including the inability to equalize the distorted optical power spectrum of an incoming WDM signal, awkward fault isolation mechanisms, network traffic congestion and inter-manufacturer incompatibility. In particular, the relative optical gain of individual optical channels in the WDM spectrum can become distorted by the effects of gain tilt in optical amplifiers located upstream from the ADM. That is to say, the initial (and usually optimal) relative intensity of wavelengths in the power spectrum of a WDM or DWDM signal is not preserved by amplifiers that provide signals to the ADM, thereby reducing the effective maximum distance between amplifiers and causing undesirable effects such as increased bit-error rate. When gain tilt affects the power spectrum of a WDM signal, prompt action must therefore be taken to equalize or restore its optimal spectral shape.
Moreover, faults occurring upstream cause a loss of data in the ADM, but it is difficult for the component to differentiate between a nonexistent signal at its input and one that has been deliberately set to zero intensity, especially since an optical ADM has no visibility into the data due to the absence of optoelectronic conversion.
Furthermore, in the event of a fault downstream from the ADM, it will continue to send data to the faulty region until personnel is dispatched to manually "drop" the traffic, i.e., reroute the traffic towards operable regions of the network.
Similarly, knowledge of a traffic bottleneck downstream may also suggest rerouting the traffic at an ADM. Unfortunately, prior art components are not equipped with a real-time controllable add/drop capability, and hence congestion remains an unavoidable phenomenon.
Another fundamental problem that plagues modern-day fiber optic network manufacturers is the high cost of interoperability. When one manufacturer links its network with that of a peer (e.g. for use during an emergency or after acquisition of the peer company), modification of equipment in one or both of the existing networks, due to, for example, differences in control signalling, may incur exorbitant costs. No solutions to this or any of the other aforementioned problems in relation to optical ADMs has been found in prior art teachings.